Farm ponds can act as greenhouse gas sinks in the Canadian Prairies

The Conversation14 May 2019, 15:38 GMT+10

Farming is considered one of the largest sources of greenhouse gases in Canada. New research suggests that small farm ponds in the Prairies actually capture nitrous oxide, a key component of farm emissions.

Climate change has sparked considerable research into what are known as the sources and sinks of greenhouse gases in natural and modified ecosystems. Aquatic ecosystems such as lakes, wetlands and ponds play a significant role in landscape greenhouse gas emissions.

Based on previous research of inland waters, we expected that water bodies in agricultural areas would be heavy producers of greenhouse gases, such as nitrous oxide, owing to the high levels of nitrogen in farm fertilizers. Instead, when we measured nitrous oxide in 101 constructed farm ponds across Saskatchewan in 2017, we found that 67 per cent of them were acting as nitrous oxide sinks.

Dissolved gases

All water bodies have some dissolved carbon dioxide, methane and nitrous oxide. When the level of these gases is higher in the water than in the atmosphere, the gas can escape, similar to the effect of opening a can of pop and letting the carbon dioxide escape as bubbles. In contrast, if these gas concentrations in the water are low, the atmospheric gases can dissolve and be taken up into the water.

Lakes have been found to be significant contributors to the global carbon cycle and, accordingly, a lot of research is currently dedicated to determining the levels and controls of the gases in water bodies. Most natural lakes and ponds tend to act as greenhouse gas sources, because bacteria in the water are breaking down organic matter and consuming nutrients which produces these gases. In some water bodies, however, these gas concentrations are low, because different biological and chemical pathways are instead consuming the gases.

Sources and sinks

Nitrous oxide in water bodies is not yet well-studied, but most previous work has observed higher levels of nitrous oxide when there are high concentrations of other forms of nitrogen such as nitrate and ammonium in the water. This often occurs in agricultural regions where these forms of nitrogen are used as fertilizer, which can then run off into lakes and ponds.

We were surprised to discover that the agricultural ponds that we studied had low levels of nitrous oxide and were acting as net sinks of this greenhouse gas. The levels of the other nitrogen forms were very high, but the majority of ponds had levels of nitrous oxide that were lower than the atmosphere.

We used statistical modelling to determine that deeper ponds, with higher growth of algae, had lower nitrous oxide. Our results suggest that the deeper regions of ponds, where oxygen levels are low, provide ideal conditions for bacteria to consume the nitrous oxide, and algae may take up the other forms of nitrogen before it can be converted into nitrous oxide.

Minimal methane

In our research, we also measured the other major greenhouse gases: carbon dioxide and methane. We have found previously that carbon dioxide is low in lakes in the Canadian Prairies, because the chemistry of the water in this region is influenced by distinct soil properties. The water tends to be alkaline in the Prairies, which causes any carbon dioxide to be chemically converted into other forms of dissolved carbon, such as bicarbonate and carbonate. We have found similar trends of alkaline waters in the farm ponds in our study, and CO2 is generally low.

Methane is often the major component of greenhouse gases produced in small water bodies, but the levels of this gas fluctuate widely over space and time. We did observe high levels of methane in many of our ponds, but intriguingly, many sites only produced minimal amounts of methane.

This meant that when adding up the greenhouse gas impact of all three gases, some sites were actually net greenhouse gas sinks.

The characteristics leading to lower emissions in farm ponds is multi-faceted. This research is ongoing, and our future analyses will seek to tease apart the factors that will provide the largest offset to reducing overall emissions.

Ponds against climate change

The ponds included in our study are constructed water bodies, and as such, have many opportunities for management with the goal of increasing greenhouse gas uptake from the atmosphere. So far, we have concluded that ponds deeper than three metres are more likely to act as nitrous oxide sinks, and those with overall better water quality have less carbon emissions.

We plan to compile construction and management recommendations when all three greenhouse gas analyses are complete. With this information, landowners may be able to add carbon offsets to their land while additionally providing water storage and resources for their farmland.